This invention relates generally to circuits for detecting existence of a given electric signal, and more particularly, is directed to improvements in a circuit for detecting existence of an information signal which is suitable for use in an apparatus for reproducing an information signal in the information signal recording and/or reproducing system such as the digital audio disc system.
In a digital audio disc system a digital audio disc which is a rotatable disc shaped recorded medium on which a digital audio signal is recorded as an information signal is provided. Such a digital audio disc is applied to a reproducing apparatus so that the digital audio signal recorded on the digital audio disc is reproduced. In such a known system, a reproduced audio signal of high quality can be obtained easily, and various different types of such a system have been already proposed. One of them is the optical digital audio disc system wherein a laser light beam, for example, is utilized to record and/or read out the digital audio signal.
In production of the digital audio disc, that is, recording of the digital audio signal, in the optical digital audio disc system, first an audio signal to be recorded is converted into a digital signal, for example, a pulse code modulation (PCM) signal modulated in a predetermined pulse code, and a laser beam modulated in intensity by this PCM signal is impinged on a recording medium provided appropriately to form a large number of pits or bumps on the recording medium in response to the variations in "1" and "0" of the PCM signal, so that a record master is produced. Then a plurality of duplicated digital audio discs are pressed by the record master.
In a manufacturing process of such a record master, the pits formed on the record master are provided in a different manner with the same PCM signal as a result of the difference in the power of the laser beam or other conditions in relation to the laser beam, and therefore it would also result that the digital audio discs duplicated from different record masters have different pit formations depending on the respective record masters. For example, digital audio discs having the different respective ratios in length of a pit to a portion next to the pit at the place where the PCM signal recorded thereat has the same pulse width for both "1" and "0" would be produced. When one of such digital audio discs is applied to a reproducing apparatus which has an optical head for reading out the recorded PCM signal from the disc by means of scanning the pits by the laser beam therefrom, it will result that the signal read out from the disc has its portions corresponding to for "1" and "0" with different time lengths, respectively, at the place on the disc where the PCM signal recorded thereat has the same pulse width for both "1" and "0". This phenomenon is called "asymmetry". In case the asymmetry arises, it is easily caused that a reproduced PCM signal obtained by wave-shaping the signal read out from the disc into a pulse train becomes different from the recorded PCM signal. This would result in the audio signal not being reproduced properly.
Accordingly, to avoid the problem mentioned above, in the reproducing apparatus in the optical digital audio disc system, a wave shaping circuit for reforming the signal read out from the digital audio disc, that is, a reproduced information signal into a pulse train, is provided with a countermeasure to the asymmetry so that the reproduced PCM signal can be properly obtained under the situation of the asymmetry. In such a reproducing apparatus, the wave shaping circuit which is operative to correct the errors induced in the reproduced information signal by the asymmetry and reform the reproduced information signal into the properly reproduced PCM signal is provided. However, when the reproduced information signal is not obtained when the disc is not rotated or the disc is not applied to the reproducing apparatus, the wave shaping circuit generates a spurious pulse signal as if it were generating to reproduced PCM signal, and the spurious pulse signal causes trouble in the reproducing apparatus such that it makes a buzz noise sound and click noise or to make a control mechanism therein operate improperly.
To avoid such a problem, it is effective to distinguish between a situation wherein the reproduced information signal is obtained and a situation wherein the reproduced information signal is not obtained, and make the reproducing apparatus inoperative and further suitably protect suitably the control mechanism in the reproducing apparatus when the reproduced information signal is not obtained.
For the purpose of making the above distinction, a circuit for detecting existence of the reproduced information signal has been provided in the reproducing apparatus. The previously proposed detecting circuit employed in the reproducing apparatus utilizes the fact that the reproduced information signal obtained from the disc has the frequency characteristic which has a peak at a particular frequency when the disc rotates at a proper rotating speed for detecting existence of the reproduced information signal. In such a detecting circuit, a frequency component having the particular frequency is extracted from the signal obtained at the output of the optical head and the level of the frequency component is measured, and as a result, it is concluded that the reproduced information signal is obtained when the level of the frequency component exceeds a predetermined reference level.
However, with the previously proposed detecting circuit mentioned above, it is very difficult to determine appropriately the predetermined reference level of the frequency component having the particular frequency, and in addition the distinction can be made only when the disc rotates at the proper rotating speed. Further, the previously proposed detecting circuit requires many parts and the circuit structure is very complicated.